Fibre optic sensors exist which can monitor events over a distance of twenty kilometers or more, and which can be operated with relatively low power. Such fibre optic sensors can detect acoustic and seismic disturbances, such as for example the footfalls of an intruder near a monitored perimeter, the noises associated with intentional damage of a monitored piece of infrastructure such as an electrical or communications cable, the noise of a leak in a pipeline, or the breaking of a reinforcing wire in a concrete pipeline or a wire in a bridge cable. Some such sensors have spaced sensing gratings, spaced by shielded portions, so that the location of a disturbance can be found by determining at which grating(s) the disturbance is noted. Others use pulsed laser light, where reflected signals caused by a disturbances are reflected back to the origin and the location from which the signals came is determined by the time lag from the pulse to the reception of the reflected signal.
Such fibre optic sensors have not been very effective, because many different types of disturbance can trigger a response. Once a response is triggered, the location from which it came must be investigated to determine whether a condition requiring corrective action is present. Further, sensors which depend on the reflection of a pulse may miss or misinterpret transient effects which have their maximum effect at a time when the pulse is not scanning the particular location where they occur.
Interferometric sensors are known which are sensitive to the measurand for a long length, for example, the entire length of the fibre optic sensor. Because the entire length, or a long length in the area of interest, is sensitive to the measurand, a signal indicating a disturbance is acquired at or very close to the source of the disturbance. This gives an advantage in signal-to-noise ratio, in that the sensor is not displaced longitudinally from the disturbance along the fibre, as is the case where there are spaced, fixed sensors. Because the distance from the nearest sensing point to the source of the disturbance is minimized, the deterioration of the signal-to-noise ratio relating from signal attenuation with distance is also minimized.
Interferometric sensors are well known in the art, and several types are known, such as a Sagnac effect interferometric sensor shown in Udd U.S. Pat. No. 5,636,021 or a Michelson interferometric sensor as shown in Jones et al U.S. Pat. No. 4,725,143
Finding the location at which the disturbance occurred along the length of an interferometric sensor is difficult. Udd (U.S. Pat. No. 5,636,021), Tapanes et. al (U.S. Pat. No. 6,621,947) and Kyoo, Juarez and Taylor ((2000) SPIE, Vol. 5090. Pp 131-141 have tried to achieve the location of the disturbance using amplitude ratios of counter-propagating beams (Udd), arrival times of disturbances in loop interferometers (Tapanes) or phase sensitive, optical time domain reflectometry (Kyoo, Jurarez and Taylor). However, the proposed ways of finding the location do not work well. In Udd, if the return loop of the Sagnac loop is affected by the disturbance, the ratiometric approach used to estimate the location does not work well, and it is often not possible to tell if the return loop is affected. In Tapanes, the slew rate of the signals arising from a disturbance makes the source location difficult.
In the case of time domain reflectometry-based methods, such as that of Kyoo, Fernades and Taylor, the location of the disturbance can be determined by seeing the point along the returned signal of a pulse where it is perturbed, or where a perturbation starts, arising from the disturbance. However, the use of a pulsed laser of this sort means that there is not continuous monitoring. Instead, each location along the fibre optic cable is only monitored at the times when a pulse passes through it. Further, especially in long sensors, there is considerable noise and only limited bandwidth is available. Continuous monitoring can be very important when one is sensing an evanescent event, or an event where the measured “signature” changes rapidly with time, making it impossible to deduce what caused the event without a complete record. Also, the reduced bandwidth often gives insufficient information to characterize the signal received, in order to assess its likely cause.
Some examples of evanescent events include:                the acoustic signal caused by the breakage of a reinforcing wire in a concrete water pipe wrapped with reinforcing wires.        the acoustic signal caused when a reinforcing wire in the bridge cable snaps.        the landing of an object, such as a thrown object, which has been thrown into a perimeter guarded by a perimeter intrusion sensing system.        
Known fibre optic sensors do not both identify the location from which a signal comes and give enough information to make a reliable identification of what is causing the signals. In this way, they are inferior to existing non-fibre-optic systems. For example, Paulson U.S. Pat. No. 5,798,457 uses acoustic or seismic detectors in an array to detect signals and analyse both the location from which the signal arises and its characteristics to see if it is indicative of a condition such as a wire break.